We believe this was caused by ebucketmigrator unknowingly did not buffer enough nacks in it's upstream direction. Causing deadlock. ebucketmigrator was waiting upstream memcached/ep-engine to eat sent nacks and memcached/ep-engine was waiting on ebucketmigrator consuming tap messages sent to it.

Aleksey Kondratenko (Inactive)
added a comment - 28/Dec/11 6:59 AM We believe this was caused by ebucketmigrator unknowingly did not buffer enough nacks in it's upstream direction. Causing deadlock. ebucketmigrator was waiting upstream memcached/ep-engine to eat sent nacks and memcached/ep-engine was waiting on ebucketmigrator consuming tap messages sent to it.
The fix is here: http://review.couchbase.org/11859 and is already merged.